Semiconductor device and method of producing same

ABSTRACT

A semiconductor device comprises a semiconductor substrate having a pillar-like portion projecting upwardly from the main surface thereof and defining circumferential side walls and a top surface, a first insulating layer covering the circumferential side walls and the upper, major surface of the substrate including the juncture therebetween. A first conductor is formed in surrounding relationship relatively to the pillar-like portion and includes integral base and upper portions extending from the juncture and upwardly to an upper edge thereof, at an intermediate position of the height of the side wall surfaces, and the base portion extending outwardly from the juncture to an outer, base edge. A second insulating layer is formed on the exposed surface of the first conductor and has upper and lower edges connected to the first insulating layer and a third insulating layer is formed on the exposed surfaces of the second insulating layer and the remaining, exposed portions of the first insulating layer and has an exposed, upper surface substantially coplanar with the flat, top surface of the pillar-like portion. A second, elongated conductor is formed on and extends along the exposed surface of the third insulating layer and includes a portion aligned with and electrically connected to the top, flat surface of the pillar-like portion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method of manufacturing same, and more particularly, to a semiconductor memory device wherein a head portion of a semiconductor pillar having a conductive side wall is brought into contact with a conductive pattern, and a method of manufacturing same.

2. Description of the Related Art

FIGS. 1 and 2, respectively, show cross-sectional views of a pillar-shaped DRAM (Dynamic Random Access Memory) and a ROM (Read Only Memory) of the prior art.

In FIG. 1 a SiO₂ film 2 is formed on a silicon substrate 1, and a semiconductor pillar 3 is formed on the SiO₂ film 2 and consists of a first conductive layer (storage electrode) 4, p⁻ type Si layer 5, channel doped layer 6, and a second conductive layer (drain) 7. The DRAM of FIG. 1 further comprises a dielectric film 8, a gate insulating film (SiO₂) 9 a second conductive layer (electrode) 10, a polysilicon word line (gate) 11, an interlaminar insulating film 12 such as a PSG film, etc., and an aluminum bit line 13.

Further, as shown in FIG. 2, the ROM likewise comprises a pillar 3 formed on a silicon substrate 1 having a n⁺ drain region 7, dielectric film 2, a gate insulating film 9, a doped polysilicon gate electrode (word line) 11, a PSG interlaminar insulting film 12, and an aluminum bit line 13. The channel region 6 and the drain region 7 form a semiconductor pillar 3 on a portion of the source region 19.

To bring the semiconductor pillar 3 into contact with the bit line 13, a contact hole 13a as shown in FIGS. 1 and 2 in the interlaminar insulating film 12, by a photolithography process. In the process of forming the contact holes 13a, the positioning of the second conductive layer 7 and the contact hole 13a (in FIG. 1) and of the drain region 7 and the contact hole 13a (FIG. 2) is necessary at the exposure, and accordingly the whole head portion of the semiconductor pillar cannot be used for the contact, and thus the diameter of the semiconductor pillar must be enlarged.

FIGS. 3A to FIGS. 3G show cross-sectional step views of the production of another DRAM cell according to the prior art.

First, as shown in FIG. 3A, a SiO₂ film 2 having a thickness of 0.5 μm, an n⁺ conductive layer 4, a p⁻ silicon layer 5 having a thickness of 5 μm, a p channel doped layer 6, and a n⁺ conductive layer 7 are formed on a silicon substrate 1.

Then, as shown in FIGS. 3B(1) a trench 15 is formed by etching the semiconductor layers (7, 6, 5 and 4) by an RIE process using an oxide film 14 as a mask. Each semiconductor pillar 3 obtained by the etching has a top surface cross section of 0.7 μm by 0.7 μm and the gap d1 between pillars in the X-direction is greater than that (d2) in the Y-direction, as shown in the top plan view of FIG. 3B(2). Then, as shown in FIG. 3C, after removing the mask 14 a SiO₂ film 17 and n⁺ poly Si film 18 are formed; thereafter, the exposed SiO₂ film 17 is etched by hydrofluoric acid so that only the non-exposed portion of the dielectric SiO₂ film 17 (i.e., the portion between the pillar 3 and the film 18) remains.

Then, as shown in FIG. 3D(1) the structure is thermally oxidized and a gate SiO₂ film 17a having a thickness of 200 Å is formed. Further, after forming a poly Si film by a CVD process on the exposed surface of the substrate, the poly Si film is etched back by an RIE process so that a side wall poly Si film 20, which becomes a gate electrode, remains; as explained above, different gaps are set up between the pillars, i.e., d1 is larger than d2 as shown in FIG. 3D(2).

Then, as shown in FIG. 3E, a SiO₂ film 17b is formed by thermal oxidation and a layer 21 of phospho-silicate-glass (PSG) or SiO₂ is deposited over the entire surface of the substrate by a CVD process.

Then, as shown in FIG. 3F, the deposited PSG or SiO₂ layer 21 is etched back until the top surface of the semiconductor pillar is exposed.

Then an aluminum film is deposited and patterned so that a bit line 22 is formed, and the bit line 22 and the n⁺ conductive layer 7 in the semiconductor pillar are connected by a self alignment process. The above description is of a DRAM realized by a prior art, and in the etching back process shown in FIG. 3F the end point of the etching, wherein the semiconductor pillar is exposed, is not easily found, and therefore, since overetching in often carried out, the SiO₂ film 17 at the upper corner position A in FIG. 3F is also etched, whereby the desired thickness of the SiO₂ film can not be maintained and the breakdown voltage is lowered.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a semiconductor device wherein a conductive pattern such as a bit line or like is brought into contact with a semiconductor pillar formed on a semiconductor substrate, by a self-alignment process, and a method of producing the semiconductor device.

Another object of the present invention is to provide a semiconductor device, such as a DRAM cell structure wherein the semiconductor pillar formed on a semiconductor substrate has a smaller diameter, and a method of producing the semiconductor device.

Therefore, according to the present invention there is provided

a semiconductor device which comprises

a semiconductor substrate having a major surface

a semiconductor pillar portion of the substrate projected upwardly from the semiconductor substrate and having side walls and a top surface, an insulating layer formed on the circumferential side wall surfaces of the semiconductor pillar and the substrate,

a conductive side wall formed on portions of the insulating layer corresponding to and at the juncture of the circumferential surface of the semiconductor pillar and the substrate, a second insulating layer formed on and covering the exposed surface of the conductive side wall, obtained by thermally oxidizing a part of the conductive side wall in such a manner that the upper portion of the further insulating layer is thicker than the remaining, lower portion thereof,

a third insulating layer formed on the substrate so that the surface of the third insulating layer surrounds the conductive side wall and is continuously and coplanar with the head a top surface portion surface of the semiconductor pillar, and

a conductive pattern in electrical contact with the head, or top surface of the semiconductor pillar and electrically insulated from the conductive side wall.

Further, there is provided a method of producing a semiconductor device, which comprises the steps of preparing semiconductor substrate, forming a semiconductor pillar projected from the semiconductor substrate,

forming a first insulating layer on a head or top surface of the semiconductor pillar,

forming a second insulating layer on the circumferential side wall surfaces of the semiconductor pillar,

forming a conductive side wall on the circumference of the semiconductor pillar, forming a third-insulating layer on the surface of the conductive side wall,

forming a fourth insulating layer on the entire exposed surface of the device so that the semiconductor pillar and the conductive side wall are buried,

removing the fourth insulating layer so as to form a coplanar surface thereof with the head surface of the semiconductor pillar,

selectively, further insulating the upper portion of the conductive side wall without further insulating the semiconductor pillar by using the first insulating layer as a mask, to improve the breakdown voltage between the conductive side wall and other elements,

removing at least the part of the first insulating layer formed on the head surface of the semiconductor pillar, and forming a conductive pattern on the head surface of the semiconductor pillar in electrical contact therewith and on the fourth insulating layer.

BRIEF DESCRIPTION FOR THE DRAWINGS

FIGS. 1 and 2 are, respectively, cross-sectional views of a pillar shaped DRAM and a pillar shaped ROM of the prior art;

FIGS. 3A to 3G are cross-sectional views of successive steps in the production of another DRAM cell according to the prior art;

FIGS. 4A to 4F are cross sectional views of an exemplary structure, as produced at successive steps of the process according to the present invention;

FIGS. 5 and 6 are cross-sectional views of a DRAM and a ROM, respectivily, according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An example of the present invention will now be explained with reference to FIGS. 4A to 4F.

FIGS. 4A to 4F show cross-sectional views of an exemplary structure as produced at successive steps of the process according to the present invention.

First, as shown in FIG. 4A, a silicon nitride (Si₃ N₄) film 24 having a thickness of about 0.15 μm is formed on a surface of a silicon substrate 1, by a usual CVD (chemical vapour deposition) process. The Si₃ N₄ film 24 acts as an antioxidant film in an oxidizing process carried out at a later process step.

Then, the upper portion of the silicon substrate 1 is etched to form a projected pillar portion; for example, a resist, pattern (not shown) is formed on a desired portion of the surface of the Si₃ N₄ film 24, and using the resist pattern as a mask, the Si₃ N₄ film 24 and the silicon substrate 1 respectively, are selectively etched. The thickness of the resulting pillar 25 of the thus-shaped silicon is, for example, about 2.0 μm. After forming the pillar shaped silicon 25 with the Si₃ N₄ film 24 on the resulting, top flat (i.e., planar) surface 25a thereof, the exposed circumferential (i.e., vertical) side wall surfaces 25b of the pillar shaped silicon 25 is oxidized to form an oxide film 27 having a thickness of about 300 Å, as shown in FIG. 4B.

Then a conductive side wall 28 is formed on the oxide film 27 of the pillar shaped silicon 25. The conductive side wall 28 is formed by the steps of depositing a polysilicon layer on the entire exposed surface, implanting n type impurities (dopant) therein, and diffusing same, and carrying out an anisotropic etching process step using a reactive ion etching so that the polysilicon material remains on only a portion of the (vertical) side wall surfaces 25b of the pillar shaped silicon 25, extending over, and from, a portion of the upper surface 1a of the substrate (as covered by the insulating layer 27) and upwardly along the vertical side wall surfaces 25a to a position adjacent to, but below, the top surface 25b whereby the polysilicon side wall 28 is formed. Then the surface of the polysilicon side wall 28 is oxidized to form an oxide film 29, and subsequently, a PSG or SiO₂ layer 30 is formed on the entire exposed surface, as an interlaminar insulating film and so that the semiconductor pillar 25 is buried as shown in FIG. 4C.

Then, as shown in FIG. 4D the PSG or SiO₂ layer 30 is etched back so that the surface of the Si₃ N₄ film 24 is exposed, with the result that the top surfaces 25a and 30a of the pillar shaped silicon 25 and the interlaminar insulating film 30, respectivily, form a continuous flat surface.

If after removing the Si₃ N₄ film 24 and corresponding, partial etching of the PSG film 30, an aluminum wiring is formed on the resulting, exposed top surface 25a of the silicon pillar 25, contact between the silicon pillar top surface 25a and the wiring can be realized by a self alignment process and without the need for mask alignment.

When the aluminum wiring is formed by such steps, however, a semiconductor element is apt to be broken due to a low breakdown voltage between the aluminum wiring and the polysilicon side wall 28. The low breakdown voltage exists because when the PSG film 30 is etched back, a portion of the PSG film 30 adjacent the top portion 28a of the polysilicon side wall 28, and subsequently a part of the top portion 29a of the oxide film 29, are etched whereby an insulating film having a thickness sufficient to withstand the breakdown voltage is not left thereat.

According to the present invention, after exposing the Si₃ N₄ film 24, and to prevent the above-mentioned over etching, an oxidizing treatment is additionally carried out since, during this oxidizing treatment, the Si₃ N₄ film 24 acts as an antioxidizing film, and thus the pillar shaped silicon 25 is not oxidized. Further, since an oxidizing specie such as oxygen diffuses through the PSG film 30 and the oxide film 29, the polysilicon side wall 28 is oxidized, extending from the top portion 28a thereof downwardly, and thus the top portion 28a of the polysilicon side wall 28 is oxidized to a depth, e.g., about 0.2 μm, sufficient to withstand the breakdown voltage, and accordingly, the oxide film 29' includes a thickened oxide portion 31 at the top of the remaining polysilicon side wall 28', as shown in FIG. 4E.

After removing the Si₃ N₄ film 24, an aluminum wiring or interconnection pattern 32 is formed on the common, flat and exposed upper surface 25a of the pillar shaped silicon 25 and the etched-back upper surface 30a' of 30, as shown in FIG. 4F. This formation of the aluminum wiring pattern 32 does not require the formation of a contact hole in the PSG film 30, and as this technique does not require a mask alignment, the head, or top, surface 25a of the pillar shaped silicon 25 can be narrowed, relatively to the width required when a hole must be formed and whereby the aluminum wiring 32 can be formed by a self alignment process. Further, a sufficient breakdown voltage property between the aluminum wiring 22 and the polysilicon side wall 28 is provided by the above-explained oxidizing treatment and particularly due to the resulting thickened oxide portion 31.

In this example, impurity doped regions are usually and optionally formed in the silicon substrate 1 and the pillar shaped-silicon 25 (corresponding to pillar 3; in FIGS. 1 and 2, for example) during the process of the present invention; further, although in this example the breakdown voltage property is enhanced by oxidizing the polysilicon side wall 28, it may be also enhanced by nitriding the polysilicon side wall 28. In such a case, a film having an anti-nitriding property, such as an oxide film, must be formed on the surface top surface 25a of the pillar-shaped silicon 25.

FIGS. 5 and 6 show cross sectional views of respective examples of a DRAM and a ROM according to the present invention. In these two examples, bit lines 13 are formed on a pillar shaped silicon 3 by a self-alignment process. Further, the upper portion of a gate insulating film (SiO₂) 9, resulting from the additional oxidizing process, is thick. The other structural elements are substantially the same as shown in FIGS. 1 and 2.

As explained above, according to the present invention, wiring can be easily brought into contact with a surface of a semiconductor pillar formed or a substrate, by a self alignment process, without positioning, with the result that a miniaturization of a device can be realized and the costs reduced. 

I claim:
 1. A semiconductor device comprising:a semiconductor substrate having an upper, major surface and a pillar-like portion, the pillar-like portion extending upwardly from said upper, major surface to a first height, said pillar-like portion having circumferential side wall surfaces of said first height, generally transverse to and forming corresponding junctures with said upper major surface, and a flat, top surface at said first height and generally parallel to said upper, major surface; a first insulating layer of uniform thickness comprising integral, upper major, sidewall and juncture portions of uniform thickness and respectively formed on and covering said upper major surfaces of said substrate, said circumferential side wall surfaces of said pillar-like portion, and said corresponding junctures therebetween; a first conductor formed on the first insulating layer in surrounding relationship relative to the pillar-like portion, the first conductor including integral and continuous base and upper portions extending, in succession, from the juncture portions and upwardly along said sidewall portions of said first insulating layer to a second height, less than the first height and defining an upper edge of said first conductor at the second height and remaining, exposed sidewall portions of the first insulating layer extending upwardly from said upper edge of said first conductor to said flat top surface of said pillar-like portion, and said base portion of said first conductor extending outwardly from each of said juncture portions of said first insulating layer and along said upper, major surface portion of said first insulating layer by a third distance and defining a bottom, outer edge of said first conductor and remaining, exposed upper major portions of said first insulating layer, said first conductor correspondingly having integral, upper and lower exposed surface portions respectively corresponding to said upper and base portions and extending from said upper edge and to said bottom, outer edge of said first conductor; a second insulating layer of integral, upper and lower portions respectively formed on and covering the upper and lower exposed surface portions of said first conductor, said second insulating layer lower portion having a second, lower edge, at said bottom, outer edge of said first conductor, connected to said upper major portion of said first insulating layer and said second insulating layer upper portion having a second, upper edge, at said upper edge of said first conductor, connected to said remaining, exposed sidewall portions of said first insulating layer, said second insulating layer upper portion being of greater thickness than the thickness of said second insulating layer lower portion, and said second insulating layer having an exposed, outer surface; a third insulating layer formed on said exposed, outer surface of said second insulating layer, said remaining, exposed sidewall and upper major portions of said first insulating layer said third insulating layer having an exposed, upper surface substantially coplanar with said flat, top surface of said pillar-like portion; and a second, elongated conductor including a first portion aligned with and formed on said top, flat surface of said pillar-like portion and electrically connected thereto, and a second portion formed on and extending along said upper, exposed surface of said third insulating layer.
 2. A semiconductor device according to claim 1, wherein said semiconductor substrate, including said pillar-like poriton thereof, comprises silicon.
 3. A semiconductor device according to claim 2, wherein said upper portion of said second insulating layer is formed of a material selected from the class consisting of silicon oxide and silicon nitride.
 4. A semiconductor device according to claim 1, wherein said first conductor comprises polysilicon.
 5. A semiconductor device according to claim 1, wherein the thickness of said upper portion of said second insulating layer is about 0.2 μm.
 6. A semiconductor device according to claim 1, wherein said device comprises a memory device.
 7. A semiconductor device according to claim 6, wherein said memory device is selected from the class consisting of DRAM and ROM memory devices.
 8. A semiconductor device according to claim 6, wherein said first conductor comprises a word line of said memory device.
 9. A semiconductor device according to claim 6, wherein said second conductor comprises a bit line of said memory device.
 10. A semiconductor device according to claim 1, wherein said first height of said pillar-like portion is approximately 2 microns and said thickness of said upper portion of said second insulating layer is approximately 0.2 microns.
 11. A semiconductor device according to claim 1, wherein:the material of said first conductor comprises polysilicon; and said second insulating layer is formed in two steps, the first step comprising thermally oxidizing a portion of the polysilicon material of said upper and base portions of said first conductor thereby to form initially on said upper and lower exposed surface portions thereof an oxide insulating layer having corresponding upper and lower portions of respective, substantially equal and uniform thicknesses and, after forming the third insulating layer and while maintaining an antioxidizing film on the top surface of said pillar portion, performing said second step, said second step comprising preferentially, further thermally oxidizing said upper portion, relatively to said base portion, of said first conductor thereby to form a gradation of said greater thickness of said second insulating layer upper portion, ranging from a maximum thickness at said upper edge thereof, substantially equal to the distance between said first and second heights to a reduced thickness at the integral connection of said second insulating layer upper and lower portions, said reduced thickness being substantially the same as said uniform thickness of said initially-formed oxide insulating layer. 